Add SharedFlow and StateFlow documentation to the Coroutines Guide

[ianbrandt]

SharedFlow, StateFlow, and their mutable counterparts have good KDoc, but they're conspicuously absent from the main https://kotlinlang.org/docs/ site. This may limit their discoverability and use.

Flow is covered (https://kotlinlang.org/docs/flow.html), but given the significant cold/hot difference, SharedFlow and StateFlow seem worthy of guide-like coverage as well.

[ianbrandt]

Related Slack thread: https://kotlinlang.slack.com/archives/C1CFAFJSK/p1632241211140700

[ianbrandt]

I don't appear to have the necessary permissions to add the "docs" label to this issue, but that would seem appropriate.